im attempting to repair two xtant 3300 c amplfiers
one the power supply supply fets have gone
one the output fets have gone

the output devices on both amps are the same
the problem i have is the power supply fets are differant in each amp and i dont know which one is correct

the amp with the blown outputs is using irfz44 fets for the power supply and this amp does power up
the other amp which will not even power up is using
smp50n06
both amps appear to be the same revision

could someone please tell me which ones are correct before i attempt to repair

Generally, you can determine if the parts are original by looking at the solder connections. Assuming that one set of parts are the originals, the parts have similar specs so either should work in the amp.

Those are generally good replacements. The 9z34 is rated for less current so it may not be quite as reliable if the amp is driven hard.

In a linear audio amp (not class D), the most important specs are voltage and current. It must be able to withstand the total rail to rail voltage. It also must be able to withstand the current output. If the current rating is equal to or greater than the originals, it should be a good replacement.